Dc to ac electric power converting apparatus

ABSTRACT

A DC-to-AC electric power converting apparatus including an invertor circuit having a plurality of switching elements, a transformer connected to the invertor circuit, and a cyclo-converter circuit for converting the frequency of the output from the transformer. A carrier signal generator is provided for generating a carrier signal of a predetermined frequency. An invertor switching circuit for controlling the switching operation of a plurality of the switching elements of the invertor circuit in synchronization with the carrier signal and switching a plurality of the switching elements of the invertor circuit in a period in which an electric current is not substantially passed through the invertor circuit; a reference voltage signal generating circuit for generating a reference voltage signal for the AC voltage to be transmitted from the cyclo-converter circuit; and a switching signal generating circuit for generating a control signal for controlling the cyclo-converter circuit in response to the reference voltage signal generated by the reference voltage signal generating circuit and the carrier signal generated by the carrier signal generator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a DC-to-AC electric power convertingapparatus for use in an AC power supply system such as an uninterruptivepower supply system. More particularly, the present invention relates toan electric power converting apparatus of a high frequency intermediatelink system in which high frequency electric power istransmitted/received via an insulating transformer.

2. Description of the Related Art

The structure of a conventional apparatus will be described withreference to FIG. 15. FIG. 15 is a block diagram of a conventionalDC-to-Ac power converting apparatus as disclosed in IEEE PESC '88Record, pp 658-663, 1988. Referring to the drawing, reference numeral 1represents an inverter circuit, 2 represents a transformer the input ofwhich is connected to the inverter circuit 1 and 3 represents acyclo-converter circuit connected to the output of the transformer 2.Reference numeral 4 represents a filter circuit connected to the outputof the cyclo-converter circuit 3 and 5 represents a current detector fordetecting the output current from the cyclo-converter circuit 3.Reference numeral 6 represents a carrier signal generator, 7 representsa reference voltage signal generating circuit and 8 represents anabsolute circuit. Reference numeral 9 represents a PWM circuit, 10represents an inverter switching circuit and 11 represents acyclo-converter switching circuit. The inverter circuit 1 comprises foursemiconductor switching devices S₁ to S₄, while the cyclo-convertercircuit 3 comprises four semiconductor switching devices S₅, S₆, S_(5A)and S_(6A). The transformer 2 is arranged in such a manner that the turnratio of the primary coil and the secondary coil is 1:2 and anintermediate tap is formed at the midpoint of the secondary coil. Thefilter circuit 4 is an LC filter circuit comprising a reactor and acapacitor. Reference numerals 12 and 13 respectively represent a DCpower source and a load circuit connected to the DC-to-AC electric powerconverting apparatus.

Then, the operation of the above-described conventional apparatus willbe described with reference to FIG. 16. As shown in the uppermostportion of FIG. 16, reference voltage signal V* in the sine waveformtransmitted from the reference voltage signal generating circuit 7 isconverted into absolute signal |V*| by the absolute circuit 8. Theabsolute signal |V*| is, together with a carrier signal transmitted fromthe carrier signal generator 6, supplied to the PWM circuit 9. As aresult, the PWM circuit 9 transmits two types binary signals T_(a) andT_(b). That is, the binary signal T_(a), the level of which is changedin synchronization with the timing at which the amplitude of theabsolute signal |V*| and that of the carrier signal are allowed tocoincide with each other, and the binary signal T_(b), the level ofwhich is changed in synchronization with the last transition of thecarrier signal, are transmitted. Then, the binary signal T_(a) and T_(b)are supplied to the inverter switching circuit 10 so that ON/OFF signalsT₁ to T₄ for switching on/off the four semiconductor switching devicesS₁ to S₄ constituting the inverter circuit 1 are transmitted. That is,the ON/OFF signals T₁ and T₃ are the same as the binary signals T_(b)and T_(a), respectively. The ON/OFF signals T₂ and T₄ are the signalsobtained by respectively inverting the sign of the binary signals T_(b)and T_(a). When the level of the ON/OFF signals T₁ to T₄ is high, thecorresponding semiconductor switching devices S₁ to S₄ are switched on.When the same is low, the corresponding semiconductor switching devicesS₁ to S₄ are switched off. As a result of the structure shown in FIG.15, the relationships among the semiconductor switching devices S₁ to S₄and the secondary voltage V₂ of the transformer 2 are expressed asfollows:

    When the switches S.sub.1 and S.sub.3 are switched on: V.sub.2= 0

    When the switches S.sub.1 and S.sub.4 are switched on: V.sub.2= V.sub.dc

    When the switches S.sub.2 and S.sub.3 are switched on: V.sub.2=- V.sub.dc

    When the switches S.sub.2 and S.sub.4 are switched on: V.sub.2= 0 (1)

where symbol V_(dc) denotes the DC output voltage from the DC powersource 12.

Therefore, when the semiconductor switching devices S₁ to S₄constituting the inverter circuit 1 are switched on/off in response tothe ON/OFF signals T₁ to T₄, V₂ becomes AC voltage the pulse width ofwhich has been modulated as shown in FIG. 16.

When the binary signal T_(b), the reference voltage signal V* and outputcurrent i_(cc) from the cyclo-converter circuit 3 transmitted from thecurrent detector 5 are supplied to the cyclo-converter switching circuit11, ON/OFF signals T₅, T₆, T_(5A) and T_(6A) for respectively switchingon/off the four semiconductor switching devices S₅, S₆, S_(5A) andS_(6A) constituting the cyclo-converter circuit 3 are transmitted fromthe cyclo-converter switching circuit 11. It is assumed that thepolarity of the output current i_(cc) is defined in such a manner thatthe direction, in which the output current i_(cc) is supplied to theload circuit 13, is positive. When the polarity of the i_(cc) ispositive, the semiconductor switching device S₅ or S₆ is switchedon/off. When the same is negative, S_(5A) or S_(6A) is switched on/off.

As a result of the structure arranged as shown in FIG. 15, therelationship between the output voltage V_(cc) from the cyclo-convertercircuit 3 and the secondary voltage V₂ of the transformer 2 is expressedas follows:

    When S.sub.5 or S.sub.5A is switched on: V.sub.cc= V.sub.2

    When S.sub.6 or S.sub.6A is switched on: V.sub.cc=- V.sub.2 ( 2)

Therefore, when the ON/OFF signal T₅ or T_(5A) is made the same as thebinary signal T_(b) and as well the ON/OFF signal T₆ or T_(6A) is madethe signal formed by inverting the sign of the binary signal T_(b), thepolarity of V_(cc) becomes positive. When the ON/OFF signal T₅ or T_(5a)is made the signal formed by inverting the sign of the binary signalT_(b) and as well the ON/OFF signal T₆ or T_(6A) is made the same as thebinary signal T_(b), the polarity of V_(cc) becomes negative. As aresult, the cyclo-converter switching circuit 11 discriminates thepolarity of the reference voltage signal V* and the output currenti_(cc) from the cyclo-converter circuit 3 respectively supplied from thereference voltage signal generating circuit 7 and the current detector5. Thus, the ON/OFF signals T₅, T₆, T_(5A) and T_(6A) as shown in FIG.16 are generated from the binary signal T_(b) supplied from the PWMcircuit 9 in accordance with the thus discriminated polarity. Inaccordance with this, sine-wave voltage, the pulse width of which hasbeen modulated and which is as shown in the lowermost portion of FIG.16, can be obtained as the output voltage V_(cc) from thecyclo-converter 3. When the obtained output voltage V_(cc) is thensupplied to the filter circuit 4, sine-wave voltage V_(L) from which thehigh frequency component has been eliminated due to the PWM operation issupplied to the load circuit 13. When the frequency of the carriersignal is raised sufficiently with respect to the frequency of thereference voltage signal V* at this time, the load voltage V_(L) to besupplied to the load circuit 13 becomes the voltage from which the highfrequency component has been sufficiently removed due to the PWMoperation and the amplitude and the phase thereof have been madesubstantially the same as those of the reference voltage signal V*. FIG.16 illustrates a switching pattern when the load circuit 13 has beenmade the linear load of the delay power factor.

Then, electric currents which respectively pass through each of theswitching elements of the inverter circuit 1 and the cyclo-convertercircuit 3 will now be considered. Since the electric current output fromthe cyclo-converter 3 continuously passes, the electric current is notturned on/off at the moment at which each of the switching elements isswitched off. As a result, it commutates to another switching elementwhich is switched on. In the cyclo-converter circuit 3, since a pair isformed by the switches S₅ and S₆ and another pair is formed by theswitches S_(5A) and S_(6A), the electric current is complementarilycommutates to each other. For example, when the switch S₅ switches offthe electric current, the current commutates to the switch S₆ which isswitched on. On the contrary, when the switch S_(5A) switches off theelectric current, the current commutates to the switch S_(6A). In theinverter circuit 1, the electric current circulates in the invertercircuit 1 when V₂₌ 0 in Equation (1). On the contrary, the electriccurrent passes through the DC power source 12 when V₂₌ V_(dc) or-V_(dc). For example, when the switch S₄ switches off the electriccurrent in a state where the electric current passes because theswitches S₁ and S₄ are switched on, the electric current commutates tothe switch S₃ which is switched on.

As described above, the conventional DC-to-AC electric power convertingapparatus receives DC electric power and transmits AC electric power inaccordance to the reference voltage signal. The above-described DC-to-ACelectric power converting apparatus is usually called "a high frequencyintermediate link type electric power converting apparatus" since thehigh frequency electric power is supplied/received via a transformer.There has been known a fact that a structure, in which the highfrequency intermediate link type electric power converting apparatus isemployed in an AC power source apparatus such as the uninterruptivepower supply system, will enable the size and the weight of theinsulating transformer and the filter circuit to be reduced.

However, the conventional DC/AC power converting apparatus encountersproblems in that the conversion efficiency is unsatisfactory and theswitching frequency cannot be raised due to large switching lossgenerated because the switching elements of the inverter circuit and thecyclo-converter circuit switch the electric current on and off.Furthermore, there arises another problem of generating voltage surgedue to switching.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide DC/ACpower converting apparatus exhibiting small switching loss, highconversion efficiency and reduced voltage surge.

According to the present invention, there is provided a DC-to-ACelectric power converting apparatus comprising: an inverter circuithaving a plurality of switching elements and converting DC electricpower into AC electric power; a transformer connected to the invertercircuit; a cyclo-converter circuit for converting the frequency of theoutput from the transformer; a carrier signal generator for generating acarrier signal of a predetermined frequency; an inverter switchingcircuit for controlling the switching operation of a plurality of theswitching elements of the inverter circuit in synchronization with thecarrier signal and switching a plurality of the switching elements ofthe inverter circuit in a period in which an electric current is notsubstantially passed through the inverter circuit; a reference voltagesignal generating circuit for generating a reference voltage signal forthe AC voltage to be transmitted from the cyclo-converter circuit; and aswitching signal generating circuit for generating a control signal forcontrolling the cyclo-converter circuit in response to the referencevoltage signal generated by the reference voltage signal generatingcircuit and the carrier signal generated by the carrier signalgenerator.

Other and further objects, features and advantages of the invention willbe appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram which illustrates a first embodiment of thepresent invention;

FIG. 2 is a block diagram which illustrates an inverter circuit, atransformer and a cyclo-converter circuit according to the firstembodiment;

FIG. 3 is a block diagram which illustrates an inverter switchingcircuit according to the first embodiment;

FIG. 4 is a block diagram which illustrates a switching signalgenerating circuit according to the first embodiment;

FIG. 5 is a timing chart which illustrates the operation of the firstembodiment;

FIG. 6 is a block diagram which illustrates a second embodiment of thepresent invention;

FIG. 7 is a block diagram which illustrates a cyclo-converter circuitand a filter circuit according to the second embodiment;

FIG. 8 is a block diagram which illustrates a first switching signalgenerating circuit according to the second embodiment;

FIG. 9 is a block diagram which illustrates a second switching signalgenerating circuit according to the second embodiment;

FIG. 10 is a timing chart which illustrates the operation of the secondembodiment;

FIG. 11 is a block diagram which illustrates a third embodiment of thepresent invention;

FIG. 12 is a block diagram which illustrates an inverter switchingcircuit according to a third embodiment;

FIG. 13 is a block diagram which illustrates a switching signalgenerating circuit according to the third embodiment;

FIG. 14 is a timing chart which illustrates the operation of the thirdembodiment;

FIG. 15 is a block diagram which illustrates a conventional DC-to-ACelectric power converting apparatus; and

FIG. 16 is a timing chart which illustrates the operation of theDC-to-Ac electric power converting apparatus shown in FIG. 15.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

FIGS. 1 to 5 illustrate a first embodiment of the present invention,where FIG. 1 illustrates the structure of the first embodiment.Referring to FIG. 1, reference numeral 2A represents a transformer, 6Arepresents a carrier signal generator, 14 represents an inverter circuitand 15 represents a cyclo-converter circuit. Reference numeral 16represents a reference voltage signal generating circuit and 17Arepresents an inverter switching circuit. Reference numeral 18Arepresents a switching signal generating circuit. The filter circuit 4,the DC power source 12 and the load circuit 13 are the same as those forthe conventional structure.

FIG. 2 illustrates the detailed structure of each of the invertercircuit 14, the transformer 2A and the cyclo-converter circuit 15. Theinverter circuit 14 comprises input terminals 141 and 142 connected tothe DC power source 12, semiconductor switching devices S₁ to S₄ such astransistors and MOSFETs, diodes D₁ to D₄ connected to the respectiveswitching devices S₁ to S₄ in an inverted parallel manner and outputterminals 143 and 144. The transformer 2A comprises primary coilterminals 21 and 22 connected to the output terminals 143 and 144 of theinverter circuit 14 and secondary coil terminals 23 and 24, thetransformer 2A having a transformation ratio of 1:1. The cyclo-convertercircuit 15 comprises input terminals 151 and 152 connected to thesecondary coil terminals 23 and 24 of the transformer 2A, semiconductorswitching devices S₅ to S₈ and S_(5A) to S_(8A) such as transistors andMOSFETs, diodes D₅ to D₈ and D_(5A) to D_(8A) connected to theabove-described switching devices S₅ to S.sub. 8 and S_(5A) to S_(8A) inan inverted parallel manner and output terminals 153 and 154 connectedto the filter circuit 4. The above-described two semiconductor switchingdevices S_(n) and S_(nA) (n=5 to 8) and diodes D_(n) and D_(nA) (n=5 to8) connected to the two semiconductor switching devices S_(n) and S_(nA)(n=5 to 8) constitute bidirectional switches each of which is arrangedin such a manner that the direction, through which electric power issupplied, can be controlled.

FIG. 3 illustrates the detailed structure of the inverter switchingcircuit 17A which comprises an input terminal 170 connected to thecarrier signal generator 6A, a peak detecting circuit 171 for detectingthe peak of the signal supplied to the input terminal 170, a 1/2 divider172 the polarity of the output signal from which is inverted insynchronization with the output from the peak detecting circuit 171, NOTcircuit 173 connected to the 1/2 divider 172 and output terminals 174 to177.

FIG. 4 illustrates the detailed structure of the switching signalgenerating circuit 18A which comprises an input terminal 200 connectedto the carrier signal generator 6A, an input terminal 201 connected tothe reference voltage signal generating circuit 16. An absolute circuit202 is provided the output of which is connected to a comparator 203constituted in such a manner which transmits an output signal having anarrow width at an intersection between the last transition slope of thecarrier signal V_(p) and the reference voltage signal |V_(cc) *|. Alsoprovided is a comparator 205 transmits an output signal having a narrowwidth at an intersection between the first transition slope of thecarrier signal V_(p) and the reference voltage signal |V_(cc) *|, NOTcircuits 207, 208 and 210, 1/2 dividers 204 and 206 the polarity of theoutput from which is inverted in synchronization with the lasttransition of the input signal, a polarity discriminating circuit 209,AND circuits 211 to 218, OR circuits 219 to 222 and output terminals 223to 226.

Then, the operation of the above-described structure will be describedwith reference to a timing chart shown in FIG. 5. First, carrier signalV_(p) in the triangular form as shown in the uppermost portion of FIG. 5is transmitted from the carrier signal generator 6A. Then, ON/OFFsignals T₁ to T₄ the duty ratio of each of which is 50% are transmittedfrom the inverter switching circuit 17A due to the following operation:referring to FIG. 3, the carrier signal V_(p) is supplied via the inputterminal 170, signal synchronized with the peak of the carrier signalV_(p), and input to the 1/2 divider 172 by the peak detecting circuit171. Signal T_(x) shown in FIG. 5 is transmitted from the 1/2 divider172, while signal T_(y) formed by inverting the sign of the signal T_(x)is transmitted from the NOT circuit 173. As a result, the signal T_(x)serving as the ON/OFF signals T₁ and T₄ are transmitted through theoutput terminals 174 and 175, while the signal T_(y) serving as theON/OFF signals T₂ and T₃ are transmitted through the output terminals176 and 177. When the level of each of the ON/OFF signals T₁ to T₄ ishigh, the corresponding semiconductor switching devices S₁ to S₄ of theinverter circuit 14 shown in FIG. 2 are switched on, while the same areswitched off when the above-described level is low. As a result of thestructure shown in FIG. 2, the relationships among the semiconductorswitching devices S₁ to S₄ and the secondary voltage V₂ of thetransformer 2A are expressed as follows:

    When the switches S.sub.1 and S.sub.4 are switched on: V.sub.2= V.sub.dc

    When the switches S.sub.2 and S.sub.3 are switched on: V.sub.2=- V.sub.dc ( 1)

where symbol V_(dc) denotes the DC output voltage from the DC powersource 12. Therefore, the secondary voltage V₂ becomes rectangular wavevoltage the duty ratio of which is 50% as shown in FIG. 5.

On the other hand, reference voltage signal V_(cc) *, which denotes thevoltage to be transmitted from the cyclo-converter circuit 15, istransmitted from the reference voltage signal generating circuit 16 soas to be supplied to the switching signal generating circuit 18Atogether with the carrier signal V_(p). When the switching signalgenerating circuit 18A receives the above-described signals V_(cc) * andV_(p), it transmits the switching signals T₅ to T₈, the pulse width ofeach of which has been modulated, as follows: referring to FIG. 4, thereference voltage signal V_(cc) * supplied through the input terminal201 is converted into absolute signal |V_(cc) *| by the absolute circuit202. The above-described absolute signal |V_(cc) *| is divided into twoportions either of which is supplied to the comparator 203 so as to besubjected to a comparison with the last transition of the carrier signalV_(p) supplied via the input terminal 200 and the other of which issupplied to the comparator 205 so as to be subjected to a comparisonwith the first transition of V_(p).

The output from the comparator 203 is transmitted to the 1/2 divider 204which then transmits a signal T_(a) shown in FIG. 5. The output from thecomparator 205 is transmitted to the 1/2 divider 206 which thentransmits signal T_(b) shown in FIG. 5. When the signal T_(a) issupplied to the NOT circuit 207, signal T_(c) is transmitted. When thesignal T_(b) is supplied to the NOT circuit 208, signal T_(d) istransmitted. Then, the relationships among the signals T_(a) to T_(d)and the output voltage V_(cc) from the cyclo-converter circuit 15 willbe described. When the polarity of the output voltage V_(cc) is desiredto be made positive, the switching signals T₅ to T₈ are determined inaccordance with the following equations:

    T.sub.5 =T.sub.a, T.sub.6 =T.sub.d, T.sub.7 =T.sub.c, T.sub.8 =T.sub.b (4)

In response to the switching signals T₅ to T₈, the semiconductorswitching devices S_(n) and S_(nA) (n=5 to 8) constituting thebi-directional switch are switched on/off. The relationships among theoperation of the semiconductor switching devices S₅ to S₈ and S_(5A) toS_(8A) and the output voltage V_(cc) from the cyclo-converter circuit 15are expressed by the following equations:

    When S.sub.5 and S.sub.8 (or S.sub.5A and S.sub.8A) is switched on: V.sub.cc =V.sub.2

    When S.sub.6 and S.sub.7 (or S.sub.6A and S.sub.7A) is switched on: V.sub.cc =-V.sub.2

    When S.sub.5 and S.sub.6 (or S.sub.5A and S.sub.6A) is switched on: V.sub.cc= 0

    When S.sub.7 and S.sub.8 (or S.sub.7A and S.sub.8A) is switched on: V.sub.cc= 0                                               (5)

Therefore, as can be shown from Equations (4) and (5), when the levelsof the signals T_(a) and T_(b) are respectively high and low, V_(cc)=V₂. When the levels of the signals T_(a) and T_(b) are respectively lowand high, V_(cc) =-V₂. When the levels of the signals T_(a) and T_(b)are simultaneously high or low, V_(cc=) 0. Therefore, the output voltageV_(cc) from the cyclo-converter circuit 15 is, as shown in FIG. 5,subjected to the PWM operation so as to be made positive voltage. On thecontrary, when the polarity of V_(cc) is desired to be made negative,the switching signals T₅ to T₈ may be determined in accordance with thefollowing equations:

    T.sub.5 =T.sub.c, T.sub.6 =T.sub.b, T.sub.7 =T.sub.a, T.sub.8 =T.sub.d (6)

The operation of the switching signal generating circuit 18A will now bedescribed. The polarity discriminating circuit 209 transmits polaritysignal V_(sgn) of the reference voltage signal V_(cc) *. Furthermore,the NOT circuit 210 transmits a signal obtained by inverting the sign ofthe polarity signal V_(sgn). The above-described signals and signalsT_(a) to T_(d) are supplied to the OR circuits 219 to 222 via the ANDcircuits 211 to 218. When the polarity of the reference voltage signalV_(cc) * is positive, the signals T_(a), T_(c), T_(d) and T_(b) aretransmitted from the corresponding AND circuits 211, 214, 216 and 217.Therefore, the output terminals 223 to 226 transmit the switching signalT₅ to T₈ which correspond to Equation (4). Similarly, when the polarityof the reference voltage signal V_(cc) * is negative, the switchingsignals T₅ to T₈ corresponding to Equation (6) are transmitted.

As a result of the above-described operation, voltage V_(cc) having thewaveform formed by pulse-width-modulating the AC reference voltagesignal V_(cc) * transmitted from the reference voltage signal generatingcircuit 16 is transmitted from the cyclo-converter circuit 15.Furthermore, the output voltage V_(cc), from which its high frequencycomponent has been removed by the filter circuit 4 connected to theoutput side of the cyclo-converter circuit 15, is supplied to the loadcircuit 13.

Then, the electric currents which respectively pass through the inverter14 and the cyclo-converter 15 will now be described with reference toFIG. 5.

Output current I_(cc) from the cyclo-converter 15 is, as shown in FIG.5, a continuous electric current which is determined by the filtercircuit 4 and the load circuit 13. The electric current which passesthrough the cyclo-converter 15 repeats the following two modes due tothe PWM operation performed by the switching element, that is, a mode inwhich it passes through the cyclo-converter 15 and another mode in whichit circulates in the cyclo-converter 15:

(i) Passing Mode

A mode in which two arms S₅ and S₈ of the cyclo-converter arms or twoarms S₆ and S₇ of the same are simultaneously turned on so that thevoltage V_(cc) is transmitted. The output current I_(cc) is transmittedfrom the inverter 14 via the cyclo-converter 15.

(ii) Circulation Mode

A mode in which two arms S₅ and S₆ of the cyclo-converter arms or twoarms S₇ and S₈ of the same are simultaneously turned on so that thevoltage V_(cc) becomes zero. The output current I_(cc) circulates in thecyclo-converter 15 before it is transmitted. Therefore, no electriccurrent passes through the inverter 14.

Therefore, as shown in FIG. 5, the input electric current I_(s) receivedby the cyclo-converter 15 passes in only a period in which the voltageV_(cc) is transmitted. The pulse width of V_(cc) is controlled in such amanner that its center is made to be the zero point of the carriersignal V_(p). Thus, its width becomes the width between two peaks ofV_(p) when the pulse width becomes largest. Therefore, by setting thegain of the reference voltage generating circuit 16 in such a mannerthat the maximum absolute value |V_(cc) *| of the reference voltagesignal shown in FIG. 5 is smaller than the peak value of the carriersignal V_(p), V_(cc) necessarily becomes zero in the vicinity of thepeak value of the carrier signal V_(p) and also I_(s) necessarilybecomes zero. On the other hand, the ON/OFF signals T_(x) and T_(y)synchronize with the peak of the carrier signal V_(p) as shown in FIG.5, switching of the inverter 14 is necessarily performed in a periodI.sub. s= 0, that is, in a period in which the electric current is notsubstantially passed through each arm of the inverter 14. The switchingloss of the switching element of the inverter 14 relates to the electriccurrent, which passes through the switching element, and the appliedvoltage. Therefore, when the level of the electric current is zero, noswitching loss is generated. That is, the inverter 14 is able to beoperated while preventing the switching loss.

Since the surge voltage which is generated due to switching of theinverter 14 and which affects the voltage resistance of the circuitelement is generated when the electric current which passes through by aquantity corresponding to the inductance of the inverter circuit 14 isinterrupted, an undesirable surge voltage cannot be generated byperforming the switching operation when the electric current, whichpasses through the switching element, is zero.

Then, a second embodiment of the present invention will be describedwith reference to FIGS. 6 to 10. According to this embodiment, athree-phase AC voltage is transmitted as an example of the cases inwhich a multi-phase AC output is obtained. FIG. 6 is a block diagramwhich illustrates the second embodiment. Referring to the drawing,reference numeral 4A represents a filter circuit, 15A represents acyclo-converter circuit and 16A represents a reference voltage signalgenerating circuit. Reference numeral 18B represents a first switchingsignal generating circuit, 30A represents a second switching signalgenerating circuit and 13A represents a three-phase load circuitconnected to the above-described DC-to-AC electric power convertingapparatus. The other elements are the same as the elements according tothe first embodiment.

FIG. 7 illustrates the detailed structure of the cyclo-converter circuit15A and that of the filter circuit 4A. The cyclo-converter circuit 15Acomprises input terminals 400 and 401 connected to the secondary coilterminals 23 and 24 of the transformer 2A, semiconductor switchingdevices S₅ to S₁₀ and S_(5A) to S_(10A) such as transistors and MOSFETs,diodes D₅ to D₁₀ and D_(5A) to D_(10A) connected to the above-describedswitching devices S₅ to S₁₀ and S_(5A) to S_(10A) in an invertedparallel manner and output terminals 402 and 404 connected to the filtercircuit 4A. The above-described two semiconductor switching devicesS_(n) and S_(nA) (n=5 to 10) and diodes D_(n) and D_(nA) (n=5 to 10)connected to the two semiconductor switching devices S_(n) and S_(nA)(n=5 to 10) constitute bidirectional switches each of which is arrangedin such a manner that the direction, through which electric power issupplied, can be controlled.

The filter circuit 4A comprises input terminals 405 to 407 respectivelyconnected to the output terminals 402 to 404 of the cyclo-convertercircuit 15A, reactors L_(F) and condensers C_(F) and output terminals408 to 410.

FIG. 8 illustrates the detailed structure of the first switching signalgenerating circuit 18B which comprises input terminals 420 to 422connected to the reference voltage signal generating circuit 16A, aninput terminal 423 connected to the carrier signal generator 6A,comparators 424 to 426, NOT circuits 430 to 432, polarity discriminatingcircuits 433 to 435 and output terminals 436 to 444.

FIG. 9 illustrates the detailed structure of the second switching signalgenerating circuit 30A which comprises input terminals 450 to 455connected to the output terminals 436 to 441 of the first switchingsignal generating circuit 18B, input terminals 456 to 458 connected tothe output terminals 442 to 444, an input terminal 459 connected to theinverter switching circuit 17, XOR (exclusive OR) circuits 462 to 470and output terminals 471 to 476.

Then, the operation of the second embodiment will be described withreference to FIG. 10. First, the triangle shape carrier signal V_(p)shown in the uppermost portion of FIG. 10 is transmitted from thecarrier signal generator 6A. The carrier signal V_(p) is then suppliedto the inverter switching circuit 17 so that the inverter switchingcircuit 17 transmits the ON/OFF signals T₁ to T₄. The four semiconductorswitching devices S₁ to S₄ of the inverter circuit 14 are switchedon/off in response to the ON/OFF signals T₁ to T₄. As a result, thesecondary voltage V₂ of the transformer 2A becomes rectangular waveformvoltage the duty ratio of which is 50% as shown in FIG. 10. Since theabove-described operation is the same as that according to the firstembodiment, the detailed description is omitted here. Then, three-phase(phases, u, v and w) AC reference voltage signals V_(ccu) *, V_(ccv) *and V_(ccw) * are transmitted from the reference voltage signalgenerating circuit 16A so as to be supplied, together with the carriersignal V_(p), to the first switching signal generating circuit 18B.

Then, the operation of the four switching devices S₅, S₆, S_(5A) andS_(6A) included in the cyclo-converter circuit 15A for controlling thevoltage of the phase u will be described with reference to FIG. 10.Referring to FIG. 8, the u-phase reference voltage signal V_(ccu) *supplied to the input terminal 420 of the first switching signalgenerating circuit 18B is, together with the carrier signal V_(p)supplied to the input terminal 423, supplied to the comparator 424. As aresult, the first switching signal T_(pu) as shown in FIG. 10 istransmitted from the comparator 424. The signal T_(pu) is supplied tothe NOT circuit 430 so that the first switching signal T_(qu) shown inFIG. 10 is transmitted. These first switching signals T_(pu) and T_(qu)are transmitted through the output terminals 436 and 437, respectively.The polarity of V_(ccu) * is discriminated by the polaritydiscriminating circuit 433 so as to be transmitted through the outputterminal 442 as u-phase voltage polarity signal V_(sgn).

Then, the operation of the second switching signal generating circuit30A will be described. Referring to FIG. 9, the u-phase voltage polaritysignal V_(sgu) transmitted from the first switching signal generatingcircuit 18B and the signal Tx, which is shown in FIG. 10, transmittedfrom the inverter switching circuit 17A are supplied to the XOR circuit462 via the input terminals 456 and 459. The XOR circuit 462 transmitsthe signal Y_(u) of a high level when the level of the polarity signalV_(sgu) and that of the signal Tx are the same (that is, the polarity ofthe u-phase output voltage V_(ccu) of the cyclo-converter 15A and thatof the secondary voltage V₂ of the transformer 2A are the same). On theother hand, the XOR circuit 462 transmits the Y_(u) signal of a lowlevel when the level of the polarity signal V_(sgn) and that of thesignal Tx are different from each other. Then, the switching signalsT_(pu) and T_(qu) transmitted from the first switching signal generatingcircuit 18B are supplied via the input terminals 450 and 451 so as to besupplied, together with the signal Y_(u), to the XOR circuits 465 and466. In response to this, the second switching signals T₅ and T₆ whichcorrespond to the polarity of the secondary voltage V₂ are transmittedfrom the XOR circuits 465 and 466 through the output terminals 471 and472. As a result, the switching devices S₅, S₆, S_(5A) and S_(6A) areswitched on/off.

The output voltage from the cyclo-converter circuit 15A will now bedescribed with reference to phase u. While arranging the electricpotential at the neutral point of the voltage supplied to thecyclo-converter circuit 15A, that is, the electric potential at themidpoint of the secondary coil of the transformer 2A to be thereference, voltage V_(uo) of the u-phase output terminal 402 isexpressed by the following equation: ##EQU1##

Since the signal T₅ for switching the switching elements S₅ and S_(5A)is generated from the PWM signal T_(pu) and the signal T_(x) denotingthe polarity of V₂ in the above-described manner, the waveform of thevoltage V_(uo) becomes as shown in FIG. 10, the waveform being formed bypulse-width modulating in such a manner that its basic wave componentbecomes V_(ccu) * corresponding to the PWM signal T_(pu).

Also the waveforms of the v-phase output voltage V_(vo) and the w-phaseoutput voltage V_(wo) become those corresponding to the referencesignals V_(ccv) * and V_(ccw) *, respectively before they aretransmitted to the v-phase output terminal 403 and the w-phase outputterminal 404, respectively. The high frequency components of the outputvoltages V_(uo), V_(vo) and V_(wo) are removed by the filter circuit 4Abefore they are transmitted to the output terminals 408 to 410 so as tobe supplied to the load circuit 13A.

Taking note of the electric current which passes through thecyclo-converter 15A, there exists a passing mode and a circulation modesimilar to the above-described first embodiment. However, since thethree-phase circuit has a plurality of arms through which the electriccurrent circulates, it circulates through the v-phase arms (S₇ andS_(7A) or S₈ and S_(8A)), the w-phase arms (S₉ and S_(9A) or S₁₀ andS_(10A)) or both the above-described arms in a case of, for example, thecirculation of the u-phase electric current I_(ccu) circulates. However,since the sum of the three-phase electric currents is always zero(I_(ccu) +I_(ccv) +I_(ccw=) 0), there is a mode in which only a portionof the u-phase electric current circulates. All of the three-phaseelectric currents circulate when the three arms S₅ (or S_(5A)), S₇ (orS_(7A)) and S₉ (or S_(9A)) are simultaneously turned on or when thethree arms S₆ (or S_(6A)), S₈ (or S_(8A)), S₁₀ (or S_(10A)) aresimultaneously turned on.

As can be seen from FIG. 10, the switching pattern in which all of thethree-phase electric currents circulate is necessarily generated in thevicinity of the peak value of the carrier signal V_(p) similarly to theabove-described first embodiment. Therefore, also according to thesecond embodiment, switching of the inverter is necessarily performed inthe period I_(s=) 0, that is, in the period, in which the electriccurrent is not substantially passed through each arm of the inverter. Asa result, a three-phase output voltage can be obtained while preventingthe generation of the switching loss in the inverter and as preventingthe surge voltage.

Then, a third embodiment of the present invention will now be describedwith reference to FIGS. 11 to 14. According to this embodiment, athree-phase AC voltage is transmitted.

FIG. 11 is a block diagram which illustrates the structure of the thirdembodiment, where reference numeral 6B represents a carrier signalgenerator for generating a sawtooth-shaped carrier signal, 17Brepresents an inverter switching circuit and 18C represents a switchingsignal generating circuit. The other structures are the same as thoseaccording to the second embodiment shown in FIG. 6.

FIG. 12 illustrates the detailed structure of the inverter switchingcircuit 17B which is structured in the same manner as the inverterswitching circuit 17A according to the first and second embodimentsexcept for the arrangement in which a detection circuit 178 is employedin place of the peak detection circuit 171, the detection circuit 178according to this embodiment acting to generate a signal at the firsttransition edge of the input signal supplied to the input terminal 170.

FIG. 13 illustrates the detailed structure of the switching signalgenerating circuit 18C which comprises the input terminals 420 to 422connected to the reference signal generating circuit 16A, the inputterminal 423 connected to the carrier signal generator 6B, comparators433 to 435 arranged to transmit output signals, each of which has anarrow width at the intersections between the last transition slope ofthe carrier signal V_(p) and the reference voltage signals V_(ccu) *,V_(ccv) * and V_(ccw) *, 1/2 dividers 427 to 429, NOT circuits 430 to432 and the output terminals 471 to 476.

Then, the operation of the third embodiment thus-constituted will now bedescribed with reference to FIG. 14.

First, the carrier signal V_(p) formed into a sawtooth shape which islowered to the right as shown in FIG. 14 is transmitted from the carriersignal generator 6B. Then, by supplying the carrier signal V_(p)thus-transmitted to the inverter switching circuit 17B, an inverterON/OFF signal T_(x) (T₁, T₄) and T_(y) (T₂, T₃) which synchronize withthe first transition edge of V_(p) is transmitted to the invertercircuit 14, causing the four switching elements S₁ to S₄ of the invertercircuit 14 to be switched on or off by corresponding signals. As aresult, the secondary voltage V₂ of the transformer 2A becomes, as shownin FIG. 14, a rectangular waveform voltage which synchronizes with thefirst transition edge of the carrier signal V_(p) and the duty ratio ofwhich is 50%. Then, the reference voltage signal generating circuit 16Atransmits the three-phase AC reference voltage signals V_(ccu) *,V_(ccv) * and V_(ccw) * before they are, together with theabove-described carrier signal V_(p), supplied to the switching signalgenerating circuit 18C.

Taking note of phase u, a narrow-width pulse is transmitted from thecomparator 433 at the intersection between the first transition slope ofthe carrier signal V_(p) and the reference voltage signal V_(ccu) *before it is divided by the 1/2 divider 427. As a result, the switchingsignal T₅ formed as shown in FIG. 14 is transmitted to the outputterminal 471. Furthermore, the signal T₆ obtained by inverting thesignal T₅ in the NOT circuit 430 is transmitted to the output terminal472. Similarly, the signals T₇ and T₈ which correspond to the phase vare transmitted to the output terminals 473 and 474 and the signals T₉and T₁₀ corresponding to the phase w are transmitted to the outputterminals 475 and 476. In this state, since the three 1/2 dividers 427to 429 are synchronized with each other, the signals T₅, T₇ and T₉respectively first-rise or trail in the same period of the carriersignal V_(p).

The switching signals T₅ to T₁₀ respectively switch on/off the switchingelement pairs composed of the switching elements S₅ -S_(5A) to S₁₀-S_(10A) of the cyclo-converter circuit 15A.

As a result, the output voltage V_(uo) formed by pulse-width-modulatingV_(ccu) * can be obtained at the u-phase output terminal 402 of thecyclo-converter circuit 15A, while the output voltage V_(vo) formed bypulse-width modulating V_(ccu) * can be obtained at the v-phase outputterminal 403 of the same. In this state, the voltage V_(uv) between thephases u and v becomes the difference between V_(uo) and V_(vo).

Similarly to the first and second embodiments, the third embodiment isenabled to have the cyclo-converter electric current passing mode andthe circulating mode. Taking note of the electric current I_(uv) whichpasses between the phase u and the phase v, the period in which thevoltage is transmitted to V_(uv) becomes the passing mode similarly tothe first embodiment. Therefore, the waveform of the uv component inI_(s) becomes as shown in FIG. 14.

As a result of a comparison made between the uv component in I_(s) andT_(x) and T_(y) shown in FIG. 14, it can be understood that theswitching of the inverter circuit is necessarily performed in the periodof the circulating mode. Since the same result can be obtained alsoconsidering the electric current passing through the phase w, all of thethree electric currents are brought into the circulating mode in thevicinity of the first transition edge of the carrier signal V_(p).Therefore, also according to the third embodiment, the switching of theinverter circuit is performed in the period I_(s=) 0, that is, in aperiod in which the electric current is not substantially passed througheach arm of the inverter circuit. As a result, the same effect as thatobtainable according to the first and second embodiments can beobtained.

Furthermore, according to the third embodiment, an effect can beobtained in that the structure can be simplified in comparison to thesecond embodiment. Although the description has been made about thethree-phase structure, the cyclo-converter can be constituted as themono-phase output similarly to the first embodiment.

Although the invention has been described in its preferred form with acertain degree of particularly, it is understood that the presentdisclosure of the preferred form has been changed in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

What is claimed is:
 1. A DC-to-AC electric power converting apparatus comprising:an invertor circuit having a plurality of switching elements and converting DC electric power into AC electric power; a transformer connected to said invertor circuit; a cyclo-converter circuit connected to said transformer; a carrier signal generator for generating a carrier signal of a predetermined frequency; an invertor switching circuit having a peak detection circuit connected to an input which detects a peak of a signal supplied to the input so that the switching operation of a plurality of the switching elements of said invertor circuit is synchronized with the carrier signal, the switching operation occurring in a period in which no electric current is passed through said invertor circuit; a reference voltage signal generating circuit for generating a reference voltage signal for AC voltage to be transmitted from said cyclo-converter circuit; and a switching signal generating circuit for generating a control signal for controlling said cyclo-converter circuit in response to said reference voltage signal generated by said reference voltage signal generating circuit and said carrier signal generated by said carrier signal generator.
 2. An apparatus according to claim 1 wherein said inverter switching circuit switches on/off a plurality of said switching elements of said inverter circuit in a period in which an output electric current from said cyclo-converter circuit circulates in said cyclo-converter circuit.
 3. An apparatus according to claim 1 wherein said inverter switching circuit transmits a control signal having a duty ratio of about 50% to said inverter circuit.
 4. An apparatus according to claim 1 wherein said cyclo-converter circuit includes a plurality of switching elements which transmit AC power which has been subjected to pulse width modulation.
 5. An apparatus according to claim 1 wherein said cyclo-converter circuit transmits multi-phase AC power.
 6. An apparatus according to claim 1 further comprising a filter circuit for removing the high frequency component of the output from said cyclo-converter circuit. 39
 7. An apparatus according to claim 1 wherein said switching signal generating circuit includes a polarity discriminating circuit connected to a reference voltage signal input which determines the sign of the reference voltage.
 8. A DC-to-AC electric power converting apparatus comprising:an invertor circuit having a plurality of switching elements and converting DC electric power into AC electric power; a transformer connected to said invertor circuit; a cyclo-converter circuit connected to said transformer; a carrier signal generator for generating a carrier signal of a predetermined frequency; an invertor switching circuit which generates a number of output signals for controlling the switching operation of a plurality of said switching elements of said invertor circuit in synchronization with the carrier signal, and switching a plurality of the switching elements of said invertor circuit in a period in which no electric current is passed through said invertor circuit; a reference voltage signal generating circuit for generating a reference voltage signal for AC voltage to be transmitted from said cyclo-converter circuit; a first switching signal generating circuit for generating a first set of switching signals in response to a reference voltage signal and a carrier signal; and a second switching signal generating circuit connected to said first switching signal generating circuit which generates a second set of switching signals responsive to the first set of switching signals, a three-phase voltage polarity signal and an output signal from said invertor switching circuit.
 9. A DC-to-AC electric power converting apparatus comprising:an invertor circuit having a plurality of switching elements and converting DC electric power into AC electric power; a transformer connected to said invertor circuit; a cyclo-converter circuit connected to said transformer; a carrier signal generator for generating a carrier signal of a predetermined frequency; an invertor switching circuit having a detection circuit connected to an input of said inverter switching circuit which generates a signal at a first transition edge of a signal supplied to the input of the inverter switching circuit so that the switching operation of a plurality of the switching elements of said invertor circuit is synchronized with a first transition edge of the carrier signal, the switching operation occurring in a period in which no electric current is passed through said invertor circuit; a reference voltage signal generating circuit for generating a three-phase reference voltage signal for AC voltage to be transmitted from said cyclo-converter circuit; a three-phase switching signal generating circuit for generating control signals for controlling said cyclo-converter circuit in response to a three-phase reference voltage signal generated by said reference voltage signal generating circuit, and the carrier signal generated by said carrier signal generator. 